1. Field of the Invention
Embodiments of the invention relate to methods, systems, and processes for desalination using reverse osmosis.
2. Background of the Related Art
Water desalination is growing to meet industrial and drinking water demand worldwide. Although both thermal desalination (multi effect distillation or “MED”, and multi stage flash evaporation or “MSF”) and membrane based seawater reverse osmosis (“SWRO”) processes are used in these plants, SWRO has grown predominantly over the last 15-20 years. SWRO has become very cost effective and efficient in terms of energy consumption as compared to where the technology was few years ago.
In conjunction with the ascendance of SWRO, there have been several developments related to low energy membranes and energy recovery devices designed to reduce energy consumption. At the same time, energy costs have been increasing more steeply, and there is a continuous need for reduced energy consumption in SWRO plants to offset the energy costs and maintain cost of water. This challenge is mostly experienced with seawater plants due to higher energy consumption, but there has been elevated attention on brackish water plants as well, due to significant increases in energy costs. These energy costs are further exacerbated by escalation of energy costs due to fouling problems during plant operation.
One challenge with plant management is that once the plant has been designed for some energy consumption, the plant's energy consumption does not remain steady and consistent once the water production starts. This may be due to several reasons, but predominantly it is because of fouling, scaling or membrane compaction. Out of these three, scaling may be the biggest contributor to energy consumption in brackish water, but fouling is the biggest cause of energy consumption in seawater and surface water-based RO plants. Moreover, due to heavy emphasis on recycling and reuse of water, it has become typical to design RO brackish water plants at as high as 97-98% recovery, which makes the fouling and scaling problems much more challenging. Sometimes the water itself may not be scaling but due to initiation that has already happened due to some other reasons scaling salts may start precipitating.
Another serious problem that RO plants encounter is bio-fouling, which reduces productivity of water, increases the differential pressure and increases power consumption. This problem is compounded in plants where there are open intakes and where water temperature increases during summer. Chlorine treatment makes this worse due to formation of oxidized products, which provide potent feed for the residual bacteria right on the membrane surface where they are rejected along with the bacteria after the de-chlorination process. Chlorination typically cannot be considered as a sustainable process option to control bio fouling, because the balance bacteria left after chlorination multiply much faster after de-chlorination with the potent nutrients as food for bacteria. Therefore it is not prudent to depend on chlorination to control bio-fouling on membranes. Moreover chlorinated organic products may be undesirable due to formation of carcinogens. Alternative techniques to control, minimize or eliminate bio-fouling are of significant interest.
Other chemical approaches like biocide treatment have found limited success and are too expensive. There have been several approaches which plants have adopted by optimizing chlorination and de-chlorination dosing, their locations and frequency including shock chlorination in the pretreatment section. These approaches have improved productivity and reduced the magnitude of this problem but have not provided a sustainable solution for plant productivity and power consumption efficiency. So there is a need to improve bio-fouling performance of SWRO and surface water and recycle reuse RO plants. Bio-fouling increases the power consumption so a low energy membrane design cannot work alone without a comprehensive approach on bio fouling control.
In an effort to maintain healthy operational efficiency in terms of water production and energy consumption, membranes should be kept in clean condition with minimum differential pressure across membranes. As the differential pressure increases it becomes difficult to clean the membranes and regain the original performance when the membrane was in its clean condition. It is also known that with higher differential pressure, permeate quality deteriorates. Beyond a point, cleaning conditions become much more aggressive and cleaning chemicals must be used for a longer time to reestablish clean membrane performance. As a matter of fact, some part of the fouling becomes irreversible and permanent. Many chemical cleanings are not practical to perform under aggressive conditions because membranes lose performance. Moreover disposal of cleaning chemicals need elaborate treatment and neutralization, which consumes additional chemicals.